1. Field of the Invention
The present invention relates to a method of thermal utilization of waste materials of different properties and origin which contain combustible or organic components, independently of the degree of contamination with heavy metals and toxic organic compounds or chlorine-containing organic compounds, while producing a clean gas which has many uses to synthesize other products and for obtaining energy, and elution-proof pure mineral residues. The method is particularly suitable for an environmentally sound utilization of waste materials, such as, domestic garbage, plastic-containing industrial wastes, paint residues, light materials obtained by shredding old automobiles, or wastes contaminated with oils.
2. Description of the Related Art
It is known in the art to burn waste materials which contain combustible components, such as, garbage, in order to utilize the energy content for producing electrical energy and for heating, to destroy toxic organic components and to reduce drastically the volume of the material to be disposed.
Incineration plants for such waste materials require a very complicated flue or smoke gas purification, especially because there is the danger that highly toxic chlorine-containing organic substances, such as, dioxins and furanes, are formed from the chlorine compounds in the material used during the combustion or in the cooling phase of the primary combustion gases. The solid residues (ash) produced during the combustion and flue dusts have a large volume, are sensitive to elution of heavy metals by atmospheric water and are difficult to dispose of. For this reason, a subsequent melting of the ash was proposed in order to convert the ash into a glass-like elution-proof slag; of course, this process is also very expensive.
In addition, it is known in the art to subject waste materials and residues to a gasification. A gasification process which can be used for this purpose is the partial oxidation with oxygen in the flow path. In this process, the fuel, residue or waste material is converted with the oxygen in the form of a flame reaction, usually also under increased pressure, into a gas which is rich in carbon monoxide and hydrogen. The process takes place at temperatures at which the mineral components already primarily form a molten slag which solidifies into a glass-like granulated slag when it is cooled and when it comes into contact with a water bath. A significant portion of the heavy metal content of the waste materials used is bound in the elution-proof granulated slag. Under the conditions of the gasification, chlorine-containing organic compounds primarily contained in the material being used are completely converted, wherein the chlorine content is converted to hydrogen chloride or non-toxic inorganic chlorides. Under these conditions, a de-novo synthesis is excluded. Consequently, the gas being produced is also free of dioxins and furanes. After mechanical purification and cooling of the sulfur content which has been practically completely converted into hydrogen sulfite, the produced gas can be used for energy purposes, for operating gas turbines and gas engines, and as synthesis gas.
The gasification in the flow path has the disadvantage that the material being used for the gasification process must be present in a flowable form, so that the material can be fed into the gasification reactor in a continuous and controlled manner. Flowable materials are gaseous and liquid materials, pumpable suspensions of finely comminuted solid materials in liquids, but also dust-like solid particles suspended in a carrier gas. However, the waste materials to be disposed of frequently have such a consistency and size that the conversion into a flowable form by mechanical processing, particularly by grinding, is technically not possible or cannot be achieved economically.
It has been proposed and tested to subject waste materials of different types to pyrolysis, i.e., a thermal conversion at temperatures of 500.degree. to 700.degree. C. Cylindrical rotary furnaces with external heating are usually used for the pyrolysis. Such furnaces have the advantage that piece-like material as well as finely particulate and flowable materials can be received. The pyrolysis produces a carbon-containing residue, a hydrocarbon-containing weak gas and tar oils which can be condensated. It has been found that the tar oils cannot be processed or are very difficult to process into fuels, heating oil or other products which can be used to synthesize other products. For the weak gas itself or for the flue gas produced during its combustion, substantial purification units are required which are similar to the units used for the flue gas purification of garbage incinerating plants. The solid residues, particularly the carbon-rich fine fractions, are difficult to dispose of, particularly because of their inflammability.
Experience in combustion technology has shown that substances contained in the material or substances newly created during the combustion as well as volatile heavy metals vaporize, are sublimated at the subsequent heating surfaces and form projections together with slag droplets suspended in the flue gas and incompletely burned coke particles, which projections cause interruptions in the operation and the continuous removal thereof also requires high investment and operating costs, and may impair the efficiency.
Moreover, as is the case in a garbage incinerating plant, it remains necessary to carry out a very substantial flue gas purification in order to limit the emission of sulfur dioxide, hydrogen chloride, nitrogen oxides and volatile heavy metals. As our own examinations have shown, even though the flue dusts separated from the flue gas are returned, it is not possible to bind volatile heavy metals, such as, cadmium, zinc, mercury or lead, in a leach-proof manner in the vitrified slag.
Therefore, it is the primary object of the present invention to provide an environmentally sound method for the thermal utilization of waste materials, wherein the method is a combination of known method steps, such as, pyrolysis, comminution, classification, gasification and gas purification. The method should make it possible to utilize piece-like, finely particulate, paste-like and liquid waste materials of different origins, which contain at least portions of combustible or organic materials and cannot be directly reused, independently of the degree of contamination with harmful materials, such as, heavy metals or toxic organic and chlorine-containing organic compounds, while producing a clean gas which can be used for synthesize other products and as an energy source and elution-proof, purely mineral solid residues which can be further utilized or simply disposed of, wherein the disposal of toxic materials in the environment, particularly also polychlorinated dibenzo dioxins and furanes, is excluded. This combination of method steps is to make possible without significant pretreatment the utilization of waste materials, such as, domestic garbage, plastic-containing industrial wastes, paint residues, old tires, light material of shredded automobiles or wastes contaminated with oils.
In accordance with the present invention, the waste materials are subjected to a carbonization or distilling process in a pyrolysis furnace with the exclusion of air at temperatures of up to approximately 800.degree. C., wherein a pyrolysis gas containing vaporized hydrocarbons and a solid pyrolysis residue are produced in the pyrolysis furnace. The pyrolysis gas is separated from the solid pyrolysis residues at temperatures above the condensation temperature of the vaporized hydrocarbons. The solid pyrolysis residues are subjected to a separation process including comminuting and classifying stages, in which a fine material is obtained which is enriched with coke-like components and a coarse material is obtained which is free of organic impurities and is composed essentially of metal components. The separated crude pyrolysis gas, the fine material and optionally an additional fluid fuel are supplied to a gasification reactor in which they are autothermally converted with a gasification agent containing free oxygen into a CO-containing and H.sub.2 -containing gas and a mineral residue, wherein the ratio of the amount of free oxygen to the amount of carbon contained in the pyrolysis gas, in the fine material and, if applicable in the additional fluid fuel, is selected in such a way that the temperatures occurring in the gasification reactor are above the melting temperature of the mineral residue and a molten slag is produced.
The present invention provides the advantage that piece-like, finely particulate, paste-like and liquid waste materials of different origins, which contain at least portions of combustible or organic materials and cannot be directly reused, can be utilized independently of the degree of the contamination with harmful materials, such as, heavy metals or toxic organic compounds and organic chlorine-containing compounds, while producing a clean gas which can be used for various purposes to synthesize other products and as an energy source and elution-proof, purely mineral solid residues which can be further utilized or simply disposed of without causing a toxic contamination, for example, by polychlorinated dibenzo dioxins and furanes. The combination of the above-described method steps, namely, pyrolysis, comminution, classification, gasification and gas purification, makes it possible without extensive pretreatment to process waste materials, such as, domestic garbage, plastic-containing industrial wastes, paint residues, old tires, light material of shredded automobiles or wastes contaminated with oil. For this purpose, the waste materials are subjected in a pyrolysis furnace to a carbonization process under the exclusion of air at temperatures of up to approximately 800.degree. C., wherein a pyrolysis gas containing vaporized hydrocarbons and a solid pyrolysis residue are produced.
An externally heated cylindrical rotary furnace may serve as the pyrolysis furnace in which the carbonization process takes place. The pyrolysis gas is separated from the solid pyrolysis residues at temperatures above the condensation temperature of the vaporized entrained hydrocarbons. The solid pyrolysis residue is subjected to a separation process which includes comminution and classification stages. The separation process produces a fine material enriched with coke-like components and a coarse material which is free of organic impurities and consists essentially of metal components.
The separated raw pyrolysis gas, the fine material mentioned above and optionally an additional fluid fuel, are fed to a gasification reactor in which they are autothermally converted with a gasification agent containing free oxygen to a gas containing CO and H.sub.2 and a mineral residue, wherein the ratio of the quantity of free oxygen relative to the quantity of carbon contained in the pyrolysis gas, and the fine material and, if applicable, the additional fluid fuel, is selected such that the resulting temperatures in the gasification reactor are greater than the melting temperature of the mineral residue and a molten slag is produced.
The molten slag is cooled, is granulated by contacting it with water and is removed from the gasification reactor.
The gas enriched with CO and H.sub.2 is cooled and sulfur compounds, hydrogen halides and aerosols are removed therefrom.
The conversion in the flow path of the gasification agent containing free oxygen takes place in the form of a flame reaction.
The fine material which is obtained in the separation process, for example, by means of screen sizing, is ground to a grain size of less than 1 millimeters, preferably less than 0.5 millimeters.
A ball mill may be used for grinding. The ground fine material is fed suspended in a carrier gas to the gasification reactor. A combustible gas serving as additional fuel may be used as the carrier.
Another possibility is to feed the fine material suspended in a carrier liquid to the gasification reactor.
The carrier liquid may be a combustible liquid which may serve as additional fuel. In this manner, it is possible to compensate for variations in the fuel content of the waste materials.
Moreover, it has been found advantageous to cool the pyrolysis gas after the separation from the solid pyrolysis residues, so that at least a portion of the vaporized entrained hydrocarbons are condensated and separated from the pyrolysis gas.
At least a portion of the hydrocarbons condensated from the pyrolysis gas can be supplied as an additional fluid fuel to the gasification reactor. Combustible waste materials from the group of pulverized solid waste materials suspended in a carrier gas, pulverized solid waste materials suspended in a carrier gas liquid, liquid waste materials and contaminated combustible gases may be fed to the gasification reactor as additional fluid fuel.
A portion of the flow of the CO-enriched and H.sub.2 -enriched gases which have been freed of sulfur compounds, hydrogen halides and aerosols may be utilized for externally heating the pyrolysis furnace.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.